1. Field of the Invention
The invention relates to a method of decoding animated images, making use of a decoder for decoding digital data relating to the images, the data being encoded in a compressed form, and having associated therewith a processor for creating images from the decoded data which are luminance and chrominance information components, and a memory for storing said information components, in which method the images are created by interpolating pixels of a preceding image and a subsequent image, while the pixels in the preceding or subsequent image may have an arbitrary position.
Such a method is particularly used in systems for transmitting images for which a moderate rate is particularly required, for example, display systems for animated images recorded on compact discs or DAT recorders, 1 Mb/s channel, and video telephones.
Algorithmic functions known in the field of image compression are used for the encoding operation, for example, discrete cosine transform, adaptive quantization, variable length encoding.
It is also possible, for example, to transmit the motion information of only one out of two frames, the others being reconstructed by symmetrical motion compensation with respect to the frames (N-1) and (N+1).
2. Description of the Related Art
As the decoding operation is performed in known manner, one out of two frames is reconstructed by interpolation using symmetrical motion compensation.
Such a real time decoding of animated images which are tightly compressed requires a large processing power as well as an extensive memory space intended for storing one or several images. The memory size is relatively important and it cannot be realizing in a single circuit integrated with the operative section of the decoder. Consequently, this causes problems of access to this external memory, as the refreshing necessitates a very considerable rate. This problem becomes even more acute when using decoders carrying out interchange encoding algorithms and reconstruction algorithms by means of frame interpolation. For generating a pixel of an image, it is necessary to have access to pixels in other images and thus to preserve several complete images in the memory. Because of the motion compensation, square or rectangular blocks instead of lines are generally used and the blocks to be processed in two different images can be moved away from each other by a distance which is equal to the amplitude of the motion to be compensated. This is the reason why the position of the pixels considered in the preceding or subsequent image may be arbitrary. It is also to be noted that it is also necessary for this decoding operation to have access to decoded pixels of another image because the encoded data may be data relating to the difference between two images.
A decoding device for realized these operations is described in an article in "IEEE Journal on Selected Areas in Communications", Vol. 6 no. 3, April 1986, pages 513, 519. This device uses a plurality of parallel arranged processors and at least four memories (MEM1 to MEM4, FIG. 3).
Such decoders are desired for use in a range of consumer products for which the low cost and the limited number of memory units are of prime importance. A significant part of the cost of such a decoder is represented by the memory. Provide a circuit which can operate with a limited number of memory units.